Re: FAILED: patch "[PATCH] sched/fair: Limit sched_cfs_period_timer() loop to avoid hard" failed to apply to 5.0-stable tree

[Phil Auld <pauld@redhat.com>]

Hi Greg,

On Tue, Apr 23, 2019 at 10:51:12PM +0200 gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 5.0-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.
>

Below, replacing the original version, is the backported version. Minor
change due to different spinlock style in the context. It should apply
to 5.0 and all the 4.x stable trees, just with increasing offsets. 

Let me know if there is anything else I can do.


Thanks,
Phil


> thanks,
> 
> greg k-h
> 
> ------------------ original commit in Linus's tree ------------------
> 
> From 2e8e19226398db8265a8e675fcc0118b9e80c9e8 Mon Sep 17 00:00:00 2001
> From: Phil Auld <pauld@redhat.com>
> Date: Tue, 19 Mar 2019 09:00:05 -0400
> Subject: [PATCH] sched/fair: Limit sched_cfs_period_timer() loop to avoid hard
>  lockup
> 
> With extremely short cfs_period_us setting on a parent task group with a large
> number of children the for loop in sched_cfs_period_timer() can run until the
> watchdog fires. There is no guarantee that the call to hrtimer_forward_now()
> will ever return 0.  The large number of children can make
> do_sched_cfs_period_timer() take longer than the period.
> 
>  NMI watchdog: Watchdog detected hard LOCKUP on cpu 24
>  RIP: 0010:tg_nop+0x0/0x10
>   <IRQ>
>   walk_tg_tree_from+0x29/0xb0
>   unthrottle_cfs_rq+0xe0/0x1a0
>   distribute_cfs_runtime+0xd3/0xf0
>   sched_cfs_period_timer+0xcb/0x160
>   ? sched_cfs_slack_timer+0xd0/0xd0
>   __hrtimer_run_queues+0xfb/0x270
>   hrtimer_interrupt+0x122/0x270
>   smp_apic_timer_interrupt+0x6a/0x140
>   apic_timer_interrupt+0xf/0x20
>   </IRQ>
> 
> To prevent this we add protection to the loop that detects when the loop has run
> too many times and scales the period and quota up, proportionally, so that the timer
> can complete before then next period expires.  This preserves the relative runtime
> quota while preventing the hard lockup.
> 
> A warning is issued reporting this state and the new values.
> 
> Signed-off-by: Phil Auld <pauld@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: <stable@vger.kernel.org>
> Cc: Anton Blanchard <anton@ozlabs.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Link: https://lkml.kernel.org/r/20190319130005.25492-1-pauld@redhat.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 310d0637fe4b..f0380229b6f2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4859,12 +4859,15 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
+extern const u64 max_cfs_quota_period;
+
 static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 {
 	struct cfs_bandwidth *cfs_b =
 		container_of(timer, struct cfs_bandwidth, period_timer);
 	int overrun;
 	int idle = 0;
+	int count = 0;
 
 	raw_spin_lock(&cfs_b->lock);
 	for (;;) {
@@ -4872,6 +4875,28 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 		if (!overrun)
 			break;
 
+		if (++count > 3) {
+			u64 new, old = ktime_to_ns(cfs_b->period);
+
+			new = (old * 147) / 128; /* ~115% */
+			new = min(new, max_cfs_quota_period);
+
+			cfs_b->period = ns_to_ktime(new);
+
+			/* since max is 1s, this is limited to 1e9^2, which fits in u64 */
+			cfs_b->quota *= new;
+			cfs_b->quota = div64_u64(cfs_b->quota, old);
+
+			pr_warn_ratelimited(
+        "cfs_period_timer[cpu%d]: period too short, scaling up (new cfs_period_us %lld, cfs_quota_us = %lld)\n",
+	                        smp_processor_id(),
+	                        div_u64(new, NSEC_PER_USEC),
+                                div_u64(cfs_b->quota, NSEC_PER_USEC));
+
+			/* reset count so we don't come right back in here */
+			count = 0;
+		}
+
 		idle = do_sched_cfs_period_timer(cfs_b, overrun);
 	}
 	if (idle)


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